NOTE! Firebase Crash Reporting will be unavailable after 9 September 2018. Crash reporting should be done by Crashlytics and Fabric. Read this Crashlytics wiki page if you need it

Complete guide is here: https://firebase.google.com/docs/android/setup

In short:

• Add the google-services classpath to the buildscript:

  buildscript{
    // ...
    dependencies {
       classpath 'com.google.gms:google-services:4.3.4'
    }
  }

• Modify android/build.gradle as follow:

  // dependencies 
  dependencies{
    implementation "pl.mk5.gdx-fireapp:gdx-fireapp-android:$gdxFireappVersion" // the version you are using
  
    implementation platform("com.google.firebase:firebase-bom:25.12.0")
    implementation 'com.google.firebase:firebase-core'
    implementation 'com.google.firebase:firebase-storage'
    implementation 'com.google.firebase:firebase-auth'
    implementation 'com.google.firebase:firebase-database'
    implementation 'com.google.firebase:firebase-crashlytics'
    implementation 'com.google.firebase:firebase-analytics'
  
    // Add this if you need google authorization
    implementation 'com.google.android.gms:play-services-auth:18.1.0'
  }
  
  // put following line at bottom of the file
  apply plugin: 'com.google.gms.google-services'

  If you have a problem with google dependencies the google maven repository is probably missing. You should add it then.

  repositories {
     maven { url 'https://maven.google.com' }
     // or google() depends of your build tools version.
  }

• Add google-services.json (it is a firebase android configuration file, you can download it from firebase console) at android/google-services.json

• That's it ! ;)